Add propertyNames validator to draft6

[torce]

I have implemented propertyNames for draft6 validator.

After executing the update_common_tests I tested it against the common tests suite and the draft6/propertyNames tests are passing.

[bastelfreak]

Hi @torce , thanks for the PR. The project was recently migrated to Vox Pupuli. Can you please rebase your branch and add tests for the change?

[codecov[bot]]

Codecov Report

Base: 89.90% // Head: 89.99% // Increases project coverage by +0.08% :tada:

Coverage data is based on head (fac9e86) compared to base (8f24e05). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #407 +/- ## ========================================== + Coverage 89.90% 89.99% +0.08% ========================================== Files 75 76 +1 Lines 1565 1579 +14 ========================================== + Hits 1407 1421 +14 Misses 158 158 ``` | [Impacted Files](https://codecov.io/gh/voxpupuli/json-schema/pull/407?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli) | Coverage Δ | | |---|---|---| | [lib/json-schema/validators/draft6.rb](https://codecov.io/gh/voxpupuli/json-schema/pull/407?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL2pzb24tc2NoZW1hL3ZhbGlkYXRvcnMvZHJhZnQ2LnJi) | `100.00% <ø> (ø)` | | | [lib/json-schema/attributes/propertynames.rb](https://codecov.io/gh/voxpupuli/json-schema/pull/407?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL2pzb24tc2NoZW1hL2F0dHJpYnV0ZXMvcHJvcGVydHluYW1lcy5yYg==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli)

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[torce]

@bastelfreak

• Solved conflicts with master branch.
• I have added a simple test for propertyNames.
• Common test suite was failing since draft7 is not supported. I have added ignore rules for all draft7 tests.

[bastelfreak]

thanks for the work!